1. Field of the Invention
The present invention relates to a process for producing a highly reliable photovoltaic element. More particularly, the present invention relates to a process for producing a highly reliable photovoltaic element excelling in characteristics by subjecting a photovoltaic element having a short-circuited current path defect therein to electrolytical treatment using a specific electrolyte solution to passivate said short-circuited current path defect present in said photovoltaic element.
2. Related Background Art
In recent years, various studies have been made in order to develop a large area photovoltaic element usable, for instance, as a solar cell for power generation. Particularly, various studies have been made in order to develop a large area photovoltaic element having a multi-layered structure comprising a semiconductor material composed of an amorphous material such as an amorphous silicon (a-Si) material. In order to produce such a large area photovoltaic element, public attention has been focused on a continuous film-forming process such as a so-called roll-to-roll film forming process.
However, it is difficult to efficiently and stably produce a large area photovoltaic element having a multi-layered structure which is free of defects such as short circuit defects in the entire region thereof. For instance, in the case of a large area thin film photovoltaic element having a stacked semiconductor structure comprising a plurality of thin semiconductor films formed of an amorphous material such as an a-Si being stacked, it is known that upon forming the stacked semiconductor structure, defects such as pinholes are liable to occur due to contamination of a foreign matter such as dust or the like in the film formed, and these defects entail shunts or short circuit defects which result in making the photovoltaic element to be markedly inferior in the characteristics required for a photovoltaic element, particularly, in the voltage-generating characteristic (in terms of the voltage component).
Herein, description will be made of the reason why such defect is occurred. For instance, in the case of an amorphous silicon photovoltaic element (or an amorphous silicon solar cell) formed on a surface of a metal substrate such as a stainless steel substrate, the substrate surface is not an entirely smooth surface but is usually provided with irregularities based on a flaw, recess, or spike-like shaped protrusion, and an electrode layer (or a back reflecting layer) having an uneven surface provided with irregularities for scattering light is often formed on the substrate surface. And on such uneven substrate surface or such uneven surface of the electrode layer, a p-type or n-type semiconductor layer comprising a thin semiconductor film having a thickness of several hundreds angstroms is formed, where it is difficult to completely cover the irregularities present at the surface of the substrate or the surface of the electrode layer and because of this, defects such as pinholes are liable to occur in the amorphous silicon photovoltaic element. In this case, pinholes are liable to occur also due to fine dusts in the film formation process.
In the case of an amorphous silicon photovoltaic element (or an amorphous silicon solar cell) comprising a lower electrode layer, a photoelectric conversion semiconductor layer comprising a plurality of amorphous silicon thin films being stacked and having such pinholes as above described, and a transparent upper electrode layer stacked in the named order on a substrate having a surface provided with a defect based on such spike-like shaped protrusion as above described, the photovoltaic element is problematic in that the photoelectric conversion semiconductor layer is defected due to the pinholes to directly contact the lower electrode layer with the transparent upper electrode layer or to contact the defect of the substrate surface. There is also a problem in that the photoelectric conversion semiconductor layer is not entirely defected but is in a state with a shunted or short-circuited portion having a low electric resistance. In this case, an electric current generated by the photoelectric conversion semiconductor layer upon the irradiation of light thereto sometimes flows in parallel to the transparent upper electrode layer to flow into a low electrically resistant portion of the shunted or short-circuited portion, where a loss in the electric current is occurred. When such loss in the electric current should be occurred, the open circuit voltage, namely, the voltage-generating characteristic, of the photovoltaic element (the solar cell) is markedly decreased. This phenomenon is more significant under condition of a low illumination intensity. This situation is seriously problematic for a solar cell for which effective power generation is required under all weather environmental condition.
For the photovoltaic element (or the solar cell) having such short-circuited portion as above described into which an electric current is flown to cause the foregoing loss in the electric current, there is a demand for diminishing the loss in the electric current as such as possible. In order to comply with this demand, there has been proposed a manner of -diminishing the loss in the electric current by directly eliminating the foregoing defects such as pinholes or eliminating or electrically insulating the peripheral material of the short-circuited portion.
Particularly, U.S. Pat. No. 4,729,970 (hereinafter referred to as document 1) discloses a manner of contacting a conversion reagent with a short circuit defect portion present in a photovoltaic device having a transparent and electrically conductive film to make a portion of the transparent and electrically conductive film in the vicinity of the short circuit defect portion to have a high electrical resistance, whereby electrically isolating the short circuit defect portion from the electrode of the photovoltaic device.
U.S. Pat. No. 5,084,400 (hereinafter referred to as document 2) discloses a manner of immersing a photovoltaic device with an electrically conductive film which is formed on a metal substrate and has a short circuit defect portion therein in a solution of an inorganic acid such as H.sub.2 SO.sub.4 or the like while impressing an electric voltage thereto to make a portion of the electrically conductive film in the vicinity of the short circuit defect portion to have a high electrical resistance, whereby electrically isolating the short circuit defect portion from the electrode of the photovoltaic device.
U.S. Pat. No. 5,320,723 (hereinafter referred to as document 3) discloses a manner of removing a short circuit defect portion present in a photoelectric conversion device by way of electrolytical treatment using an electrolyte solution containing an inorganic or organic acid, an inorganic or organic base, or a metal salt.
Besides these, it is known that in order to improve the utilization efficiency of light in a photovoltaic element comprising a photoelectric conversion semiconductor layer formed on a substrate, light arrived at the substrate through the photoelectric conversion semiconductor layer is reflected by means of a metal layer as a back reflecting layer formed between the substrate and the photoelectric conversion semiconductor layer to return it into the photoelectric conversion semiconductor layer.
It is known that the metal layer is constituted by a metallic material having a high reflectance such as Ag. However, for Ag used in this case, it is known that Ag is readily reacted with moisture to cause an Ag dendritic crystal growth which results in entailing a shunt in the photovoltaic element. In this respect, the metal layer as the back reflecting layer is usually constituted by an aluminum material.
It is also known that in order to prolong the optical path length for light in the photoelectric conversion semiconductor layer by way of light reflection, a transparent and electrically conductive layer comprising ZnO or the like and having an uneven surface is disposed between the back reflecting layer and the photoelectric conversion semiconductor layer.
However, in the case of removing a defect such as short circuit defect portion present in a photovoltaic element comprising a multi-layered photoelectric conversion semiconductor layer and having the back reflecting layer composed of the aluminum material and/or the transparent and electrically conductive layer by any of the above described defect-removing manners, such problems as will be described are liable to occur.
In the case of the manner described in document 1, when a conversion reagent containing a salt of a Lewis acid and an amphoteric element, particularly, a solution containing a chloride salt such as AlCl.sub.3, ZnCl.sub.2 or the like is used, an amphoteric metal such as Al is liable to significantly corrode into the back reflecting Al layer, resulting in entailing a problem such that side effects such as layer peeling, for example, at the interface between the Al layer and the ZnO layer is occurred.
In the case of the manner described in document 2, there is a problem in that when the electrolyte solution is used for a long period of time, the acid component thereof is concentrated so that to control so as to conduct stable reaction is difficult. There is also a problem in that when the acid concentration of the electrolyte solution is controlled so as to effectively work in removing a short circuit defect portion present in a photovoltaic element having a multi-layered lower electrode layer comprising, for instance, a metal layer and a transparent and electrically conductive layer comprising ZnO or the like, the transparent and electrically conductive layer is liable to corrode.
Similarly in the case of the manner described in document 3, there is a problem in that when the electrolyte solution contains chlorine ion, side effects such as layer peeling, for example, at the interface between the Al layer and the ZnO layer is liable to occur.